Some types of power tools such as electric screwdrivers, electric drills, electric circular saws and electric jigsaws incorporate a mechanism which is adapted to forcibly turn off a power switch for an electric motor when overload is imposed on a driver bit, drill bit or the like to produce excessive torque between the bit or the like and the output shaft of the electric motor.
Such a mechanism must meet the following requirements. First, the mechanism is required to forcibly turn off the power switch of the motor on the occurrence of overload, even if an operating member or lever is held in such a position as to rotate the motor. Preferably, when the power switch is turned off, the mechanism simultaneously produces dynamic braking force, for example, by forming a short circuit across armature coils.
Secondly, the mechanism must be so designed that when the operating member or level is once shifted to its inoperative position and then returned to its operative position, the power switch of the motor may be turned on by means of the operating lever shifted to the operative position.
The conventional mechanism of this type includes a circuit shown in FIG. 21. The circuit has two switches 1 and 3 connected between a power source 2 and a motor 4. The switch 1 is a starting switch for the motor adapted to be changed over in direct association with operation of the operating lever. The switch 1 has a contact c which is connected to a contact a and disconnected from the other contact b, when the operating lever is in its operative position. On the other hand, when the operating lever is in its operative position, the contact c is disconnected from the contact a and connected to the contact b. The switch 3 is a locking switch adapted to be changed over in operative association with an overload sensing mechanism (not shown). The switch 3 has a contact f which is, with no overload sensed, connected to a contact d and disconnected from the other contact e. On the other hand, when overload is sensed, the contact f is disconnected from the contact d and connected to the contact e.
When the operating lever is shifted to the operative position, with no overload sensed, driving current is supplied to the motor 4 through the connection of the contacts a-c and d-f. Then, if overload is sensed, the contact f is disconnected from the contact d to shut off supply of the driving current and, at the same time, the contact f is connected to the contact e to produce a dynamic braking force to be applied to the motor 4. At this time, even if the operating lever is held in the operative position, the above operation is achieved.
The locking switch 3 may be of the type in which when overload is sensed by the overload sensing mechanism, the contact f is disconnected from the contact d, and when the operating lever is shifted to the inoperative position, the contact f is brought into contact with the contact d. In this case, if the operating lever is once shifted to the inoperative position and then returned to the operative position, driving current is supplied to the motor 4 to rotate the same.
Thus, the conventional power tool having an overload cutoff feature includes two switches, that is, a starting switch (switch 1 in FIG. 21) for rotating the motor and a locking switch (switch 3 in FIG. 21) for controlling the torque. This results in increase of the number of parts as well as more intricate wiring and assembling, causing increase of manufacturing costs. Furthermore, the structure tends to be large and consequently requires greater mounting space, so that the power tool may become larger and heavier.